1. Field of the Invention
The present invention relates to an LED lamp including a wavelength converting portion with a phosphor.
2. Description of the Related Art
White LED lamps are recently under vigorous research and development as potential replacements for white incandescent lamps. In some of those white LED lamps, the package of a blue LED chip, made of gallium nitride (GaN), is coated with a phosphor such as YAG. In such an LED lamp, the blue LED chip produces an emission with a wavelength of about 450 nm, and the phosphor produces yellow fluorescence with a peak wavelength of about 550 nm on receiving that emission. Eventually, the emission and fluorescence mix with each other, thereby providing white light.
In another type of white LED lamp currently under development, an LED chip that emits an ultraviolet ray is combined with a phosphor that produces red (R), green (G) and blue (B) light rays. In such an LED lamp, the ultraviolet ray that has been radiated from the LED chip excites the phosphor, thereby emitting the red, green and blue light rays. Consequently, white light can also be obtained as a mixture of these light rays.
A bullet-shaped package is extensively used in conventional LED lamps. Hereinafter, such an LED lamp with a bullet-shaped appearance will be described with reference to FIG. 1.
FIG. 1 illustrates a cross-sectional structure for a conventional LED lamp 20 as disclosed in Japanese Patent No. 2998696, for example. As shown in FIG. 1, the LED lamp 20 includes an LED chip 21, a bullet-shaped transparent housing to cover the LED chip 21, and leads 22a and 22b to supply current to the LED chip 21. A cup reflector 23 for reflecting the emission of the LED chip 21 in the direction indicated by the arrow D is provided for the mount portion of the lead 22b. The inner walls (i.e., reflective surfaces) of the cup reflector 23 surround the side surfaces of the LED chip 21 so as to define a predetermined tilt angle with respect to the bottom of the cup reflector 23. The LED chip 21 on the mount portion is encapsulated with a first resin portion 24, which is further encapsulated with a second resin portion 25.
The first resin portion 24 is obtained by filling the cup reflector 23 with a resin material and curing it after the LED chip 21 has been mounted onto the bottom of the cup reflector 23 and then has had its cathode and anode electrodes electrically connected to the leads 22a and 22b by way of wires. A phosphor 26 is dispersed in the first resin portion 24 so as to be excited with the light A that has been emitted from the LED chip 21. The excited phosphor 26 produces fluorescence (which will be referred to herein as “light B”) that has a longer wavelength than the light A. This LED lamp 20 is designed such that if the light A radiated from the LED chip 21 is red, then the light B emitted from the phosphor 26 is yellow. A portion of the light A is transmitted through the first resin portion 24 including the phosphor 26. As a result, light C as a mixture of the light A and light B is used as illumination light. The light A may exhibit a narrow-band spectral distribution with a peak wavelength of about 470 nm, while the light B may exhibit a broad-band spectral distribution with a peak wavelength of about 570 nm, for example. FIG. 2 shows an exemplary spectral distribution of illumination light obtained from such an LED lamp.
In the conventional LED lamp shown in FIG. 1, however, if the amount of the current supplied to the LED chip (i.e., drive current) changes, a variation (such as color shifting) easily occurs in the spectral distribution of the illumination light. Such color shifting happens because the emission state of the LED chip, and eventually the spectral distribution of the light A radiated from the LED chip, are changeable with the amount of the drive current supplied thereto. Normally, the smaller the amount of drive current supplied, the longer the emission peak wavelength of an LED chip tends to be. If the spectral distribution of the light A radiated from the LED chip changes in this manner, then the spectral distribution of the light B, produced from the phosphor 26 in the resin that covers the LED chip, also changes. As a result, the spectral distribution profile shown in FIG. 2 changes. On the other hand, the luminance of an illumination source needs to be adjusted according to the situation. Accordingly, when an LED lamp is used as an illumination source, the drive current of its LED chip must be changed to achieve a desired luminance. Consequently, color shifting, resulting from such a change in the amount of drive current supplied, needs to be eliminated in order to use an LED lamp as an illumination source.